I see on my mapped QUAD, some gray diagonal lines over the map.
How can I eliminate those lines?

Polygon smooth with blendfunc (SRC_ALPHA, SRC_ALPHA_MINUS_ONE) creates
seams along quad diagonals, because quads are broken into two triangles
for rendering. You can get around this by using blendfunc
(SRC_ALPHA_SATURATE, GL_ONE), as described at
http://www.edm2.com/0603/opengl.html

The problem with POLYGON_SMOOTH (or any other approach that relies on
blending) is that it doesn't work well with the Z buffer.
Depth-independent transparency is a non-trivial problem (consider what
happens with intersecting objects.) There may be some good solutions
for certain cases, but it depends on what you need to draw in your
scene. For example the border suggestions I gave earlier work well for
2D, but not 3D.

Yes, Alex gave me a solution, that is the FSAA, but it requires 4x of
VRAM
and fill rate, so I suppose it should slow down the animation speed.
If you
instead think it's good for animations too, please let me know, so I
will go
to study that topic (I already gave it a look and it seems to be
hard...).

FSAA is a good general solution because it works with any scene and
doesn't require blending. It is also very easy to set up, just add the
sample buffers attribute to your pixel format and enable multisampling.
See this Q&A: http://developer.apple.com/qa/qa2001/qa1268.html

The downside of FSAA is that the quality is relatively low for the
added VRAM/fillrate overhead. My scenes require compositing multiple 2D
framebuffers, so FSAA wasn't the best solution for me. It might be fine
for you, it depends on what you're trying to draw.

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